This invention relates to hydraulic tensioners used as a control device for a power transmission chain drive in automotive timing applications and more particularly to a hydraulic tensioner having a long maximum useful stroke, while having a normal cylinder-piston assembly length in the retracted state.
A tensioning device, such as a hydraulic tensioner, is used as a control device for a power transmission chain, or similar power transmission device, as a chain travels between a plurality of sprockets. As a chain transmits power from a driving sprocket to a driven sprocket, one portion or strand of the chain between the sprockets will be tight while the other portion of the chain will be slack. In order to impart and maintain a certain degree of tension in the slack portion of the chain, a hydraulic tensioner provides a piston that presses against a tensioner arm or other chain guiding mechanism.
Prevention of excess slack in the chain is particularly important in the case of a chain driven camshaft in an internal combustion engine because a chain without sufficient tension can skip a tooth or otherwise throw off the camshaft timing, possibly causing damage or rendering the engine inoperative. However, in the harsh environment of an internal combustion engine, various factors can cause fluctuations in the chain tension.
For instance, wide variations in temperature and thermal expansion coefficients among the various parts of the engine can cause the chain tension to vary between excessively high or low levels. During prolonged use, wear to the components of the power transmission can cause a decrease in chain tension. In addition, camshaft and crankshaft induced torsional vibrations cause considerable variations in chain tension. Reverse rotation of an engine, occurring for example in stopping or in failed attempts at starting, can also cause fluctuations in chain tension. For these reasons, a mechanism such as a hydraulic tensioner is desired to ensure the necessary tension on the slack side of the chain.
Typically, a hydraulic tensioner includes a piston in the form of a hollow cylinder. The piston slides within a bore in the housing and is biased outward from the housing in the direction of the tensioner arm and chain by a piston spring. The interior of the piston forms a high pressure fluid chamber with the bore or opening in the housing. The high pressure chamber is connected through a one way check valve to a low pressure chamber or reservoir, which provides or is connected to an exterior source of hydraulic fluid.
Upon start-up, the force of the spring on the piston causes the piston to move further outward as the chain begins to move. Outward movement of the piston creates a low pressure condition in the high pressure fluid chamber, or pressure differential across the inlet check valve. Accordingly, the inlet check valve opens and permits the flow of fluid from the reservoir, or low pressure chamber, into the high pressure chamber. When the high pressure chamber is sufficiently filled with fluid, the force on the chain that moves the piston inward will be balanced by the outward force from the spring and the resistance force of the fluid in the chamber. The force of the chain against the fluid in the chamber also causes the check valve to close, which prevents further addition of fluid to the chamber.
U.S. Pat. No. 4,826,470 to Breon et al. discloses a cylinder/piston assembly tensioning device, in which the cylinder forms a chamber for pressurized fluid. A static plunger is interposed between the cylinder and the piston, and a spring is interposed between a flange of the static plunger and the piston. The usable stroke of the piston is limited to the extension of the piston skirt, which must never completely leave the cylinder. This patent also describes a piston formed by two parts that create between them a path for the fluid to exit. The maximum useful stroke of the piston is nevertheless limited by the axial length of the skirt integral with the piston head.
U.S. Pat. No. 4,963,121 to Himura et al. also discloses a tensioning device for a belt or chain drive mechanism that includes a hollow piston element slidable inside a stationary cylinder. A fluid chamber is defined between two reciprocally sliding tubular elements of which one is fixed to the cylinder and the other moves under the action of a fluid and of a spring, integrally with the piston. The maximum useful outward stroke of the piston is defined by the length of the skirt integral with the piston head.